Submits a promotional template for operator approval. It starts as in_progress
(in operator review) and becomes approved (usable in campaigns) or rejected with
a rejection_reason. If the submission to the operator fails, the template stays
pending and submission_error explains why — fix it and update the template.
sender_id must be a non-masking number assigned to your account. Use ##name##
placeholders for per-recipient values.

Statuses: draft, pending (not yet submitted), in_progress (in operator review),
approved, rejected, terminated / disabled (no longer usable).